Screens for cathode ray tubes with discrete phosphor layers

ABSTRACT

A screen for use in a cathode ray tube comprises two discrete phosphors of the same or different type applied on the face plate, one of the phosphors being exposed to the vacuum in the cathode ray tube whereas the other covered by an electroconductive layer. When irradiated with electron beams of different speeds the screen displays different colors. When the exposed phosphor is constructed to have high resistance the screen can act as a storage screen. A metal back layer may be provided to cover both exposed phosphor and the electroconductive layer.

United States Patent Takita 1451 Mar. 21, 1972 SCREENS F )R CATHODE RAY TUBES WITH DISCRETE PHOSPHOR LAYERS Foreign Application Priority Data' July 23, 1968 Japan ..43/52030 July 23, 1968 Japan ..43/5203l .3 :L X 31968 t 23:: US. Cl ..3l3/92 R, 313/68 A 1111.01... ..H0lj 29/32, 1101; 31/68 Field of Search ..3l3/92 PF, 92 PH, 68

[56] References Cited UNITED STATES PATENTS 3,242,260 3/1966 Cooper et al. l 78/5.4

3,271,610 9/1966 Law 3,339,099 8/1967 Anderson ..313 6s Primary Examiner-Robert Sega] Attorney-Chittick, Pfund, Birch, Samuels & Gauthier [57] ABSTRACT A screen for use in a cathode ray tube comprises two discrete phosphors of the same or different type applied on the face plate, one of the phosphors being exposed to the vacuum in the cathode ray tube whereas the other covered by an elecf troconductive layer. When irradiated with electron beams of different speeds the screen displays different colors. When the exposed phosphor is constructed to have high resistance the screen can act as a storage screen. A metal back layer may be 1 provided to cover both exposed phosphor and the electroeon- I ductive layer.

6 Claims, 13 Drawing Figures PAIENTEDMAR21 I972 3, 651 ,362

- SHEET 1 or 3 INVENTOR HAJIME TAKITA M ATTORNEIYS PATENTEDMAR21 I972 SHEET 2 0F 3 FIG. 50

FIG. 5d

wanna u/runa 'ulunu INVENTOR HAJIME' TAKITA BY J v EVA? mom SCREENS FOR CATI'IODE RAY TUBES WITH DISCRETE PHOSPI-IOR LAYERS BACKGROUND OF THE INVENTION This invention relates to a screen for a cathode ray tube. Conventional screens can be classified into an ordinary screen which luminesces when irradiated with an electron beam, a color screen wherein various types of phosphors are applied to different portions on the surface of the screen and the electron beam is selectively irradiated to vary the luminescence color and a screen for use in a so-called direct viewing storage tube wherein the electron beam is used to store electric charge on an insulator near the screen.

Screens now widely used in color television receivers are generally of the type for displaying all colors such as the shadow mask type, chromatron type and the like. However, for use in cathode ray tubes acting as oscilloscopes or data displayers, it is not necessary to provide all color displays and there are many applications which require displays of different colors instead of all color displays.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a novel screen for cathode ray tubes capable of providing luminescence, display of different colors and charge accumulatron.

According to one aspect of this invention there is provided a screen comprising a face plate, two discrete phosphors applied on the surface of the face plate, one of these phosphors being exposed to the vacuum in the cathode ray tube, and an electroconductive layer overlying the other phosphor. These phosphors may be of the same type or different type which emanate different colors when irradiated with an electron beam. One of the phosphors is applied in the form of independent islands or stripes and the other phosphor is applied to surround the islands or stripes. The islands or stripes are exposed to the vacuum in the cathode ray tube and the other phosphor is covered by an electroconductive layer. A metal back layer may be provided to cover both islands or stripes and the electroconductive layer. The phosphor underlying the electroconductive layer luminesces when irradiated with a high speed write electron beam, while the electroconductive layer prevents a low speed read out electron beam from penetrating there through and also acts to collect secondary electrons emitted by the screen.

According to the other aspect of this invention, displays of different colors are provided by irradiating the screen constructed as above described with electron beams of different speeds.

According to yet another aspect of this invention the phosphor applied in the form of islands or stripes has high resistance whereby the screen can act as a storage screen.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. I is an enlarged plan view of a portion of a screen embodying this invention;

FIG. 2 is a sectional view of the screen shown in FIG. 1 taken along a line IIlI thereof;

FIG. 3 shows an enlarged plan view of a portion of a modified screen;

FIG. 4 is a sectional view of the screen shown in FIG. 3 taken along a line IV-IV thereof;

FIGS. 5a to 5f show various steps of a method of fabricating a screen of this invention; I I

FIG. 6 shows a sectional view of a screen provided with a metal back layer;

FIG. 7 is a diagram to show an electrical connection-for operating the screen of this invention and FIG. 8 shows a secondary electron emission characteristic to explain the operation of the screen constructed according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2 of the accompanying drawing there is shown a screen embodying this invention and comprising a substrate 4 which is the same as that utilized in a conventional cathode ray tube. The substrate is fabricated from a light transmissive electric insulating material such as glass and comprises the face plate of the cathode ray tube. The substrate 4 is coated with discrete phosphors 2 and 3 of the same or different type. One of the phosphors 3 is coated in a mesh like configuration or coated as a film having a plurality of not coated openings. The other phosphor 2 is applied to fill openings in the mesh or to constitute a plurality of independent islands. A thin layer 1 of electroconductive substance such as aluminum is applied on the mesh like phosphor layer 3.

In a modification shown in FIGS. 3 and 4 one phosphor layer 8 is applied on a glass substrate 10 in the form of stripes and the other phosphor layer 9 is applied to surround the phosphor layer 8 and is covered by a thin layer of aluminum 7. In both embodiments phosphors 2 and 8 are directly exposed to the vacuum in the cathode ray tube and aluminum layers 1 and 7 are connected to a source of potential (not shown) as will be described later in more detail. Electron beams (not shown) impinge upon the surface of the screen in the direction indicated by arrows 5 and 11, respectively.

The operation of the screen when it is impinged upon by an electron beam is as follows. Firstly, it is assumed that the screen operates as a color screen. In the embodiment shown in FIGS. 1 and 2, two types of phosphors emanating two different colors are used. For example, the phosphor layer 3 beneath electroconductive layer 1 is comprised by a phosphor that emanates red color whereas the phosphor layer 2 which is exposed to vacuum is comprised by a phosphor that emanates green color. The electroconductive layer 1 serves as a screen electrode and is supplied with a positive potential with respect to the cathode electrode, not shown. Upon irradiation with an electron beam in the direction of arrows 5 (FIG. 2) phosphors 2 and 3 emanate their inherent colors. However, when the accelerating voltage for the electron beam is low, the electron beam can not penetrate through electroconductive layer 1 so that phosphor layer 3 underlying the same can not luminesce. Whereas the other phosphor layer 2 luminesces because it is exposed to vacuum to directly receive the electron beam. Thus, with low acceleration voltage for the electron beam the luminescence is only the green color emanating from phosphor layer 2 directly exposed to the electron beam. Upon increase of the acceleration voltage for the electron beam, the electron beam can penetrate through electroconductive layer 1 to excite underlying phosphor layer 3 to cause it to emanate red color.

As the electron beam accelerated by a high voltage also impinges upon the other phosphor layer 2 exposed to vacuum the electron beam causes the phosphor layer 2 to emanate green color. In this manner, the screen emanates mixed color of red and green, that is yellowish green, yellow or orange.

More specifically, where aluminum layer 1 has a thickness of about 50 m., the electron beam accelerated by a low voltage up to about 2,000 volts emanates green light, but as the voltage is gradually increased beyond 2,000 volts, the color gradually changes from green to yellowish green, yellow, orange until finally red color will be emanated. This is because that while the intensity of red increases with voltage whereas that of green does not increase with the accelerating voltage. As the surface of green phosphor layer 2 is exposed to the vacuum, in the higher voltage range its potential will drop to a level wherein the secondary electron emission ratio is unity with the result that even when the screen potential is increased beyond that level, the surface potential can not follow the potential applied to the screen. Accordingly, the luminous intensity can not increase accordingly but saturates finally. This means that the difference in the accelerating voltage for the electron beam may be small in order to display different colors.

Instead of applying different accelerating voltages to an electron beam emitted from a single electron gun, two electron beams of different velocity emitted from independent electron guns may be used for producing different colors.

In the actual operation, too wide phosphor layers 2 and 3 results in an offense to the eye. To avoid this difficulty, it is advantageous to select the width of phosphor layers 2 and 3 to be a fraction of the diameter of the focused electron beam spot impinging upon the screen. Then, the electron beam focused on the screens causes several blocks of phosphor layers 2 and 3 to luminesce at the same time, thus obviating above described difficulty.

From the above described principle of operation, it will be understood that in addition to the constructions shown in FIGS. 1 to 4, the screen may take various forms. For example, a uniform metal back layer may be applied upon the screen shown in FIGS. 1 and 2 or FIGS. 3 and 4 as will be described later in more detail. Alternatively, three or more types of phosphor layers may be applied with electroconductive overlayers of different thickness thus emanating various colors.

The operation of the novel screen acting as a direct viewing storage screen will now be briefly considered.

Phosphor layers 2 exposed to vacuum can store electric charge because they are essentially an electric insulator. When an electron beam is caused to scan across the screen under a very low accelerating voltage, the electron beam that arrives at the electroconductive layer 1 does not cause underlying phosphor layer 3 to luminesce but that arrives at exposed phosphor layer 2 charges up its surface thus accumulating electric charge thereon. If the phosphor layer is highly conductive, the charge accumulated thereon will drain toward surrounding electroconductive layer 1 so that the phosphor layer 2 should have a high resistance such for example as zinc silicate phosphor. Alternatively, a conventional phosphor may be coated to a thickness slightly thinner than is normally adequate to isolate individual particles of the phosphor, thus increasing the overall resistance of the phosphor layer, or a highly insulative substance such as magnesium oxide or silicon oxide may be incorporated into the phosphor to increase the resistance thereof, thus decreasing draining of the electric charge stored on the surface.

When an electron beam accelerated by a high voltage impinges upon a screen having electric charge stored thereon, the electron beam will drive off the stored electric charge to decrease the surface potential of the irradiated portions to a potential substantially the same as that applied to the electroconductive layer. Secondary electrons that have been driven off are collected by the electroconductive layer 1. Under these conditions, portions of the surface of the exposed phosphor layer 2 which have not been irradiated with the electron beam accelerated by high voltage still continue to preserve electric charge whereas irradiated portions lose their electric charge to manifest a potential substantially the same as that applied to the electroconductive layer 1. In other words, the electron beam accelerated by a high voltage operates to write a pattern on a uniformely charged screen. If an electron beam accelerated by a voltage which is not so high to drive off the stored electric charge from the surface of phosphor layer 2 but is not so low as to store electric charge on the surface of phosphor layer 2 which has already been written, that is a read out beam is caused to uniformly irradiate the surface of the screen, portions of the surface with the storage of electric charge will not luminesce because the electron beam can not reach them whereas portions with any electric charge will luminesce by the irradiation of the electron beam. In other words, the pattern previously written is read out. If an electron beam accelerated by a lower voltage is caused to irradiate the screen the charge will be uniformly stored even in the portions that have been written to erase the pattern thus preparing for the next write operation.

In this manner, it is possible to'write in and read out informations from the screen of this invention. Let us consider the luminous phenenoma thereof in more detail.

During writing, as the high speed electron beam reaches the screen, the electron beam arriving at the electroconductive layer 1 penetrates therethrough to cause underlying phosphor layer 3 to luminesce. Concurrently therewith, as the phosphor layer 2 exposed to high vacuum is also caused to luminesce, mixed color of two phosphor layers 2 and 3 will be resulted. It is assumed that the phosphor layer 2 is comprised by a phosphor that emanates green color and that phosphor layer 3 by a phosphor that emanates red color. Then the color emanated by writing is a mixed color of green and red, that is yellowish green, yellow or orange. On the other hand, during read out operation, since only the exposed phosphor layer 2 is caused to luminesce, the screen manifests green color.

In this manner, patterns of different colors are displayed during writing and read out operations.

Where it is not desired to provide displays of different colors, the same phosphor may be utilized to fabricate both phosphor layer 3 underlying the electroconductive layer 1 and phosphor layer 2 exposed to vacuum.

Typical example of the method of preparing the novel screen will now be described.

For the sake of description, like the above embodiments it is assumed that a phosphor R that emanates red color is used to fabricate the phosphor layer 3 underlying the electroconductive layer 1 while a phosphor G that emanates green color is used to fabricate the exposed phosphor layer 2.

FIG. 5 shows various steps of fabricating the screen. At first the phosphor R is admixed with a photosensitive material which can be set by light such as KMER and the mixture is then applied onto the surface of a well cleaned face plate or substrate 13 (FIG. 5a). After drying the applied film 14 a striped vapor depositing mask 15 is closely applied onto the dried film 14 as shown in FIG. 5b, and then ultraviolet rays are irradiated in the direction shown by arrows 17 to set portions of the film 14 corresponding to the configuration of the mask. While retaining in position the vapor depositing mask 15, aluminum is vacuum deposited in the direction of arrows 17 as shown in FIG. 50. After removing the mask 15, the screen is developed to obtain a screen comprising face plate 13 and a R phosphor layer 14 which is covered by an aluminum layer 18, as shown in FIG. 5d, then the other phosphor G is admixed with a high setting photosensitive material such as KMER and the mixture is applied on the resulted screen as shown by reference numeral 19 in FIG. 5e. In this case, it is desirable to incorporate a suitable quantity of an insulator such as magnesium oxide and silicon oxide to the photosensitive material in addition to phosphor G. After thorough drying, ultraviolet rays 20 are irradiated from the side opposite to the photosensitive material as shown in FIG. 5e. In this case it is not necessary to use a mask for shielding ultraviolet rays, because R phosphor layer 14 and overlying aluminum layer 18 which have already been formed act as a shielding mask. When developed a screen as shown in FIG. 5f can be obtained wherein gaps between R phosphor layers 14 are filled with G phosphor, thus completing a screen for a cathode ray tube.

In sealing the screen in an evacuated cathode ray tube the portions of the photosensitive material that has been set in the previous stage are decomposed by heating them in air and the alumimum electroconductive layer is treated with Aquadug and the like so that it can be supplied with voltage.

The screen constructed in accordance with this invention is advantageous in that it can be used as a color screen and also as a direct viewing accumulating screen. Further, when the screen is used as the accumulating screen, the accelerating voltage for the electron beam should be different for writing and read out operations but this difference in the accelerating voltage for the electron beam can be directly utilized to provide two colored display. More particularly, red display for example can be provided during writing whereas yellowish green during read out, this eliminates any particular operations normally required for two colored displays, in other words, it is not necessary to apply a voltage of some kind to any electrode. The other advantages is that it is not required to provide any mesh in the vacuum such as a storage mesh, and a secondary electron collecting mesh and the like as in the case of a well known directly viewing accumulating tube, thus greatly simplifying construction and fabrication. Further, the electroconductive layer not only acts as a screen electrode but also to pass an electron beam which is accelerated by a voltage beyond a predetermined value. Further, the electroconductive layer acts as a collector electrode for collecting secondary electrons as well as a reflector for light rays emanated by the underlyin g phosphor layer.

As has been already pointed out, a metal back layer may be applied to the screen of this invention in the same manner as in the conventional cathode ray tube without causing any change in its operation. FIG. 6 illustrates a cross-sectional view of such a modified screen. As shown in FIG. 6, a uniform metal back layer 23 is applied on the phosphor layer 2 and the electroconductive layer 1. Provision of the metal back layer 23 is effective to reflect forwardly through the face plate light rays emanated from phosphor layer 2 thus improving brightness. On the other hand, it becomes impossible to expect the saturation of the intensity of the luminescence when the accelerating voltage acting upon the electron beam impinging upon phosphor layer is increased. Where the metal back layer is provided, said electroconductive layer 1 may be made ofan insulator.

FIGS. 7 and 8 show the operation of the novel screen when it is used as an accumulating screen. For this purpose the screen is connected as shown in FIG. 7. For the sake of simplicity, the electron lens, deflection system, collimeter electrode, vacuum envelope as well as other component elements of the cathode ray tube have been omitted from the drawing. A grid electrode 36 is disposed to substantially surround a cathode electrode 35 and the grid bias is varied by manipulating a switch 37. Upon throwing switch 37 to one contact 37a the grid bias will become zero thus permitting an electron beam to scan across the screen, whereas when the switch 37 is thrown to the opposite contact 37b, a negative bias will be supplied to the grid from a source of grid voltage 38 of the magnitude sufficient to cut off the electron beam.

Turning now to FIG. 8 illustrating the secondary electron emission characteristic, the ordinate 43 represents the percentage of the secondary electron emission. The cross point with the abscissa 44 corresponds to zero per cent whereas a straight line 42 represents unity secondary electron emission. The abscissa 44 represents the accelerating voltage for the electron beam, while curve 48 represents a secondary electron emission curve which intersects straight line 42 of the unity secondary electron emission at several points. The first cross point 39 represents a stable point, the second cross point 40 a nonstable point, and the third cross point 41 a stable point, the voltages at respective cross points being indicated by V V and V respectively. Assuming that voltage V and V are impressed upon the electroconductive layer 1, then the third cross point 41 will shift to points 41' and 41" respectively while the secondary electron emission curve charges to curves 48' and 48" respectively. Voltage V selected for the purpose of explanation is a voltage intermediate voltages V and V and voltages V and V are voltages intermediate voltages V and V Generally, V equals from zero to several hundred volts and V and V equal to from several hundreds to several thousand volts but vary dependent upon the substance utilized as the secondary electron emission surface (in this invention, the material from which phosphor layer 2 is fabricated.) or binder and the like factors.

While maintaining the grid electrode 36 at the cut off state, when the voltage of a source 34 connected to the screen is gradually increased, the potential of phosphor layers 2 and 3 is also increased correspondly. When the voltage applied to electroconductive layer 1 is not so high, for example represented by voltage V shown in FIG. 8 and while maintaining the grid bias at zero volt, by irradiating the electron beam velocity and hence unable to cause underlying phosphor layer 3 to luminesce. On the other hand, the electron beam that reaches exposed phosphor layer 2 can not emit more than one secondary electron because of its low speed. For this reason, the surface potential of phosphor layer 2 decreases along arrow 45 shown in FIG. 8 to reach stable point 39. Accordingly, electric charge will be accumulated on the surface of exposed phosphor layer 2.

However, if phosphor layer 2 is highly conductive the electric charge will drain toward surrounding electroconductive layer 1 so that it is necessary to incorporate insulating material such as magnesium oxide or silicon oxide or to apply a conventional phosphor to lesser thickness.

While maintaining the grid electrode at the cut off state, when the screen voltage is increased to v the potential of the screen will be increased to a higher level while preserving the stored electric charge. Thus when the electron beam is projected upon a portion of the screen while the grid electrode 36 is being biased to zero, the electron beam arriving at the electroconductive layer passes therethrough to excite underlying phosphor layer 3 to cause it to luminesce. On the other hand, the electron beam that arrives at phosphor layer 2 on which electric charge has been stored acts to emit more than one secondary electrons from phosphor layer 2 because of its increased accelerating voltage. Thus causing the surface potential to settle to potential 4l' which is substantially the same as V Secondary electrons thus emitted are collected by the electroconductive layer 1. Under these conditions phosphor layer 2 of course luminesces. Whereas portions of the surface of phosphor layer 2 not irradiated with the electron beam accelerated by high voltage V; are still storing the charge so their potential is low.

Since the electron beam accelerated by this high voltage V, functions to vary the surface potential of phosphor layer 2, such an electron beam is generally termed a a writing beam. Although a pattern is formed on the surface of the exposed phosphor layer 2, it is impossible to view the pattern. To continuously read out this pattern it is necessary to uniformly project an electron beam accelerated by lower voltage V: upon the screen. To accomplish this, the grid electrode is biased to the cut off and the voltage of screen source 34 applied to electroconductive layer 1 is reduced to V When the bias voltage of grid electrode 36 is reduced to zero to irradiate the screen with a uniform low speed electron beam, the electron beam arrived at electroconductive layer 1 does not cause underlying phosphor layer 3 to luminesce because of its low accelerating voltage. Portions of the exposed phosphor layer 2 which have not been written with the high speed electrons are still storing surface charge and hence have low potential than V so that they do not luminesce when irradiated with such low speed electron beam. However, portions of the surface of the phosphor layer 2 which have been written are at a little higher potential than V the low speed-electron beam causes these portions to luminesce. Thus; the written pattern is read out. This pattern can persist when the low speed electron beam is continuously irradiated. Even when irradiation of the low speed electron beam is ceased the charge accumulated on the surface of phosphor layer 2 is preserved without decay so that when the phosphor layer 2 is again irradiated with the electron beam accelerated by voltage V; the luminescence corresponding to the stored pattern can be obtained again.

As mentioned above, the irradiation by the low speed electron beam may be effected by utilizing the electron beam emitted from the same electron gun utilized for writing. Alternatively, in addition to the electron gun for writing, an endependent gun may be provided to emit an electron beam operating at low speed. Such a low speed electron beam is generally termed as a flood beam or a read out beam."

A method of erasing the written pattern will be described hereunder. While continuing irradiation of the flood beam, if

the voltage applied to the electroconductive layer 1 of the screen is reduced to a value less than V at which the secondary electron emission ratio decreases to a value less than unity the voltage of the screen that has been written will be decreased accordingly. For this reason the electrons of the flood beam will uniformly adhere to the surface of the screen until finally the surface potential thereof decreases to substantially the same potential as that of the cathode which emits the flood beam. Then the written pattern will be completely erased to prepare for the next writing operation.

What is claimed is: l. A screen for cathode ray tubes which comprises in combination:

a face plate; at least a first and a second phosphor coating, said first phosphor coating applied at the surface of said face plate in the form of a discontinuous layer, said second phosphor coating applied at the surface of said face plate constituting a layer surrounding said first phosphor coatmg; and electroconductive layer overlying one of said phosphor coatings while the other of said phosphor coatings is exposed to the vacuum in said cathode ray tube, and a metal back layer overlying said exposed phosphor coating and said electroconductive layer.

2. The screen according to claim 1 wherein said two first and second phosphor coatings are of the same type.

3. The screen according to claim 1 wherein said first and second phosphor coating are of different types, emanating different colors when irradiated with an electron beam.

4. The screen according to claim 1 wherein said first phosphor coating is applied in the form of independent islands.

5. The screen according to claim 1 wherein said first phosphor coating is applied in the form of stripes.

6. The screen according to claim 1 wherein said exposed phosphor coating comprises a high electro-resistive material toform a direct viewing storage screen and said electroconductive layer is supplied with a positive potential with respect to the potential of the cathode of said cathode ray tube. 

1. A screen for cathode ray tubes which comprises in combination: a face plate; at least a first and a second phosphor coating, said first phosphor coating applied at the surface of said face plate in the form of a discontinuous layer, said second phosphor coating applied at the surface of said face plate constituting a layer surrounding said first phosphor coating; and electroconductive layer overlying one of said phosphor coatings while the other of said phosphor coatings is exposed to the vacuum in said cathode ray tube, and a metal back layer overlying said exposed phosphor coating and said electroconductive layer.
 2. The screen according to claim 1 wherein said two first and second phosphor coatings are of the same type.
 3. The screen according to claim 1 wherein said first and second phosphor coating are of different types, emanating different colors when irradiated with an electron beam.
 4. The screen according to claim 1 wherein said first phosphor coating is applied in tHe form of independent islands.
 5. The screen according to claim 1 wherein said first phosphor coating is applied in the form of stripes.
 6. The screen according to claim 1 wherein said exposed phosphor coating comprises a high electro-resistive material to form a direct viewing storage screen and said electroconductive layer is supplied with a positive potential with respect to the potential of the cathode of said cathode ray tube. 